static libs can't be added to soong product_variables eng build flavor exclusively

[Dean Wheatley]

Per https://cs.android.com/android/platform/superproject/+/master:build/soong/android/variable.go;bpv=1;bpt=1 , static_libs can be declared in debuggable only, not eng. What is the reason for restricting this? I'd like to include static_libs for eng build only.